(1) Field of the Invention
The present invention relates to a gas duct comprising a ceramic honeycomb structure, used mainly in an exhaust gas purification system for automobile.
(2) Description of Related Art
Currently, gas ducts comprising a honeycomb structure are in extensive use because they are low in pressure loss (when an exhaust gas is passed therethrough) owing to the high porosity and show an excellent exhaust gas purifiability. As an example of such gas ducts, there is widely known a ceramic honeycomb catalytic converter used in an exhaust gas purification system for automobile; and it is disclosed in, for example, JP-A-49-72173 and JP-A-7-77036.
In such ducts comprising a ceramic honeycomb structure, a ceramic honeycomb structure is fitted to a gas duct in a state that it is accommodated in a metal case, in order to allow the easy handling of the ceramic honeycomb structure. At this time, a holding member made of, for example, a ceramic fiber mat is allowed to be present, in a compressed state, between the outer surface of the honeycomb structure and the inner surface of the metal case, in order to reliably hold the honeycomb structure in the metal case and also lessen the impact applied from outside.
As the method for accommodating a honeycomb structure in a metal case via a holding member, there are generally known three methods, i.e. a stuffing method, a tourniquet method and a clamshell method. The stuffing method is shown in FIG. 2(a) and comprises winding a holding member 1 round a ceramic honeycomb structure 2 and forcing the resulting material in a metal case 3 from its one opening. In this method, as shown in FIGS. 2(b) and 2(c), the two ends of the holding member 1 have to-be-connected areas 13 engageable to each other; the holding member 1 is wound round the outer surface of the honeycomb structure 2 and the to-be-connected areas 13 of the two ends of the honeycomb structure 1 are engaged to each other and fixed. The compression of the holding member 1 is conducted by, as shown in FIG. 2(d), forcing the honeycomb structure 2 covered with the holding member 1, in the metal case 3 using an insertion-assisting jig 5 of ring shape having such an inner diameter as decreases gradually from one end of the ring to the other end.
The tourniquet method comprises winding a holding member 1 round a honeycomb structure 2 as shown in FIGS. 3(a) and 3(b), inserting the resulting material into a metal case 3, placing the resulting material in between upper and lower wire ropes 18 as shown in FIG. 7, pulling the ropes upward and downward at a given load to clamp the case 3 and resultantly compress the holding member 1, thereby fixing the honeycomb structure 2 in the metal case 3.
The clamshell method comprises winding a holding member round a honeycomb structure, placing the resulting material in one pair of opposing metallic half shells having a shape symmetric to each other, and welding the half shells to each other.
As the regulation for exhaust gas emission has become stricter recently in connection with the environmental protection and, for example, a lower level has come to be required for the total hydrocarbon emission in the LA-4 mode which is one of the exhaust gas evaluation tests in U.S.A, ceramic honeycomb catalysts are desired to exhibit exhaust gas purifiability which is higher than before. Catalysts are not sufficiently heated and therefore are not sufficiently activated and the purification efficiency is significantly low, at the start of engine, i.e. the cold start. Thus, the early activation of catalyst at cold start is considered to be the most important task for achieving the regulation for exhaust gas emission. From this standpoint, there was made a proposal of (1) making the partition wall of ceramic honeycomb catalyst as thin as possible and making the open frontal area of the honeycomb catalyst as high as possible to reduce the pressure loss and (2) reducing the weight of honeycomb structure and lowering the heat capacity of catalyst to increase the temperature elevation rate of catalyst. In this proposal, since a large geometrical surface area is obtainable, a honeycomb catalyst of small size can be produced. From such a standpoint, there has recently been developed a ceramic honeycomb structure having thin partition walls of 0.03 to 0.10 mm in thickness
In a ceramic honeycomb structure having thin partition walls, however, it is difficult to achieve the minimum guaranteed value (10 kgf/cm2) for the isostatic fracture strength which is an index of the strength of structure. Herein, xe2x80x9cisostatic strengthxe2x80x9d is specified in the JASO standard M 505-87 which is a standard for automobile issued by Society of Automotive Engineers of Japan, Inc., and is expressed as a load at which fracture appears when an isostatic hydrostatic load is applied to a honeycomb structure.
Therefore, in the gas duct comprising a honeycomb structure, when a honeycomb structure having thin partition walls is accommodated in a metal case according to a conventional method, there has been a problem in that in the canning operation of accommodating the honeycomb structure in the metal case via a holding member, the honeycomb structure is fractured by the tourniquet of the holding member.
In view of the above-mentioned situation, the present invention aims at providing a gas duct comprising a ceramic honeycomb structure, wherein even when the ceramic honeycomb structure has thin partition walls, the honeycomb structure is not fractured when it is accommodated in a metal case, i.e. during the canning.
The presently claimed invention provides a gas duct having a ceramic honeycomb structure, with the following: a metal case, a ceramic honeycomb structure in the metal case; and a holding member between the outer surface of the ceramic honeycomb structure and the inner surface of the metal case. The holding member has two engageable ends and when the holding member is wound round the outer surface of the honeycomb structure the two ends engage each other. This engagement occurs in a connection area. The connection area and its vicinity face a plurality of partition walls of the honeycomb structure.
In the above gas duct, the honeycomb structure may be accommodated in the metal case by winding the holding member round the honeycomb structure and then forcing the resulting material in the metal case from one opening of the metal case.
In the above gas duct, each to-be-connected area of the holding member preferably has, in the winding direction, a length of 20 to 50 mm or of 5 to 15% based on the length of the holding member in the winding direction.
The present invention further provides a gas duct having a ceramic honeycomb structure, which comprises:
a metal case,
a ceramic honeycomb structure accommodated in the metal case, and
a holding member placed between the outer surface of the ceramic honeycomb structure and the inner surface of the metal case,
wherein the holding member is wound round the outer surface of the honeycomb structure,
the metal case is formed by winding a metal plate round the holding member in such a way that the two ends of the metal plate are overlapped with each other and then tourniquet the metal plate, and
the vicinity of the inner end of the overlapped two ends is allowed to face the partition wall of each cell constituting the honeycomb structure.
In the gas duct of the present invention, the partition walls of the ceramic honeycomb structure may have a thickness of less than 0.1 mm. Also, the cells of the ceramic honeycomb structure preferably have a tetragonal sectional shape. Also, the honeycomb structure may be a catalyst for exhaust gas purification.
Further in the gas duct of the present invention, the holding member is preferably a mat made of a ceramic fiber. Also, the pressure generated when the holding member is compressed, is, at a temperature range at which the gas duct is in actual use, preferably less than two times the pressure at normal temperature.